CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (1) Beamer, Summer 2007 © UCB Scott Beamer, Instructor inst.eecs.berkeley.edu/~cs61c CS61C : Machine Structures Lecture #9 – MIPS Logical & Shift Ops, and Instruction Representation I acquires for $625M

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (2) Beamer, Summer 2007 © UCB Review Functions called with jal, return with jr $ra. The stack is your friend: Use it to save anything you need. Just be sure to leave it the way you found it. Instructions we know so far Arithmetic: add, addi, sub, addu, addiu, subu Memory: lw, sw, lb, sb, lbu Decision: beq, bne, slt, slti, sltu, sltiu Unconditional Branches (Jumps): j, jal, jr Registers we know so far All of them! There are CONVENTIONS when calling procedures!

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (3) Beamer, Summer 2007 © UCB Bitwise Operations Up until now, we’ve done arithmetic ( add, sub,addi ), memory access ( lw and sw ), and branches and jumps. All of these instructions view contents of register as a single quantity (such as a signed or unsigned integer) New Perspective: View register as 32 raw bits rather than as a single 32-bit number Since registers are composed of 32 bits, we may want to access individual bits (or groups of bits) rather than the whole. Introduce two new classes of instructions: Logical & Shift Ops

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (4) Beamer, Summer 2007 © UCB Logical Operators (1/3) Two basic logical operators: AND: outputs 1 only if both inputs are 1 OR: outputs 1 if at least one input is 1 Truth Table: standard table listing all possible combinations of inputs and resultant output for each. E.g., A B A AND B A OR B

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (5) Beamer, Summer 2007 © UCB Logical Operators (2/3) Logical Instruction Syntax: 1 2,3,4 where 1) operation name 2) register that will receive value 3) first operand (register) 4) second operand (register) or immediate (numerical constant) In general, can define them to accept > 2 inputs, but in the case of MIPS assembly, these accept exactly 2 inputs and produce 1 output Again, rigid syntax, simpler hardware

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (6) Beamer, Summer 2007 © UCB Logical Operators (3/3) Instruction Names: and, or : Both of these expect the third argument to be a register andi, ori : Both of these expect the third argument to be an immediate MIPS Logical Operators are all bitwise, meaning that bit 0 of the output is produced by the respective bit 0’s of the inputs, bit 1 by the bit 1’s, etc. C: Bitwise AND is & (e.g., z = x & y; ) C: Bitwise OR is | (e.g., z = x | y; )

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (7) Beamer, Summer 2007 © UCB Uses for Logical Operators (1/3) Note that and ing a bit with 0 produces a 0 at the output while and ing a bit with 1 produces the original bit. This can be used to create a mask. Example: The result of and ing these: mask: mask last 12 bits

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (8) Beamer, Summer 2007 © UCB Uses for Logical Operators (2/3) The second bitstring in the example is called a mask. It is used to isolate the rightmost 12 bits of the first bitstring by masking out the rest of the string (e.g. setting it to all 0s). Thus, the and operator can be used to set certain portions of a bitstring to 0s, while leaving the rest alone. In particular, if the first bitstring in the above example were in $t0, then the following instruction would mask it: andi $t0,$t0,0xFFF

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (9) Beamer, Summer 2007 © UCB Uses for Logical Operators (3/3) Similarly, note that or ing a bit with 1 produces a 1 at the output while or ing a bit with 0 produces the original bit. This can be used to force certain bits of a string to 1s. For example, if $t0 contains 0x , then after this instruction: ori$t0, $t0, 0xFFFF … $t0 contains 0x1234FFFF (e.g. the high-order 16 bits are untouched, while the low-order 16 bits are forced to 1s).

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (10) Beamer, Summer 2007 © UCB Shift Instructions (1/4) Move (shift) all the bits in a word to the left or right by a number of bits. Example: shift right by 8 bits Example: shift left by 8 bits

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (11) Beamer, Summer 2007 © UCB Shift Instructions (2/4) Shift Instruction Syntax: 1 2,3,4 where 1) operation name 2) register that will receive value 3) first operand (register) 4) shift amount (constant < 32) MIPS shift instructions: 1. sll (shift left logical): shifts left and fills emptied bits with 0s 2. srl (shift right logical): shifts right and fills emptied bits with 0s 3. sra (shift right arithmetic): shifts right and fills emptied bits by sign extending

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (12) Beamer, Summer 2007 © UCB Shift Instructions (3/4) Example: shift right arith by 8 bits Example: shift right arith by 8 bits

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (13) Beamer, Summer 2007 © UCB Shift Instructions (4/4) Since shifting may be faster than multiplication, a good compiler usually notices when C code multiplies by a power of 2 and compiles it to a shift instruction: a *= 8; (in C) would compile to: sll $s0,$s0,3 (in MIPS) Likewise, shift right to divide by powers of 2 remember to use sra

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (14) Beamer, Summer 2007 © UCB What does r have to push on the stack before “jal e”? 1: 1 of ($s0,$sp,$v0,$t0,$a0,$ra) 2: 2 of ($s0,$sp,$v0,$t0,$a0,$ra) 3: 3 of ($s0,$sp,$v0,$t0,$a0,$ra) 4: 4 of ($s0,$sp,$v0,$t0,$a0,$ra) 5: 5 of ($s0,$sp,$v0,$t0,$a0,$ra) 6: 6 of ($s0,$sp,$v0,$t0,$a0,$ra) 7: 0 of ($s0,$sp,$v0,$t0,$a0,$ra) Peer Instruction r:... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem... ### PUSH REGISTER(S) TO STACK? jal e # Call e... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to caller of r e:... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to r

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (15) Beamer, Summer 2007 © UCB What does r have to push on the stack before “jal e”? 1: 1 of ($s0,$sp,$v0,$t0,$a0,$ra) 2: 2 of ($s0,$sp,$v0,$t0,$a0,$ra) 3: 3 of ($s0,$sp,$v0,$t0,$a0,$ra) 4: 4 of ($s0,$sp,$v0,$t0,$a0,$ra) 5: 5 of ($s0,$sp,$v0,$t0,$a0,$ra) 6: 6 of ($s0,$sp,$v0,$t0,$a0,$ra) 7: 0 of ($s0,$sp,$v0,$t0,$a0,$ra) Peer Instruction Answer r:... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem... ### PUSH REGISTER(S) TO STACK? jal e # Call e... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to caller of r e:... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to r Volatile! -- need to pushSaved e can’t return changed, no need to push e can return changed

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (16) Beamer, Summer 2007 © UCB Administrivia Go to your assigned lab Lab 101 is too crowded Assignments Proj1 due 11:59pm HW4 due 11:59pm Anything else?

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (17) Beamer, Summer 2007 © UCB Overview – Instruction Representation Big idea: stored program consequences of stored program Instructions as numbers Instruction encoding MIPS instruction format for Add instructions MIPS instruction format for Immediate, Data transfer instructions

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (18) Beamer, Summer 2007 © UCB 61C Levels of Representation (abstractions) High Level Language Program (e.g., C) Assembly Language Program (e.g.,MIPS) Machine Language Program (MIPS) Hardware Architecture Description (e.g., block diagrams) Compiler Assembler Machine Interpretation temp = v[k]; v[k] = v[k+1]; v[k+1] = temp; lw $t0, 0($2) lw $t1, 4($2) sw $t1, 0($2) sw $t0, 4($2) Logic Circuit Description (Circuit Schematic Diagrams) Architecture Implementation Register File AL U

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (19) Beamer, Summer 2007 © UCB Big Idea: Stored-Program Concept Computers built on 2 key principles: 1) Instructions are represented as numbers. 2) Therefore, entire programs can be stored in memory to be read or written just like numbers (data). Simplifies SW/HW of computer systems: Memory technology for data also used for programs

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (20) Beamer, Summer 2007 © UCB Consequence #1: Everything Addressed Since all instructions and data are stored in memory as numbers, everything has a memory address: instructions, data words both branches and jumps use these C pointers are just memory addresses: they can point to anything in memory Unconstrained use of addresses can lead to nasty bugs; up to you in C; limits in Java One register keeps address of instruction being executed: “Program Counter” (PC) Basically a pointer to memory: Intel calls it Instruction Address Pointer, a better name

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (21) Beamer, Summer 2007 © UCB Consequence #2: Binary Compatibility Programs are distributed in binary form Programs bound to specific instruction set Different version for Macintoshes and PCs New machines want to run old programs (“binaries”) as well as programs compiled to new instructions Leads to instruction set evolving over time Selection of Intel 8086 in 1981 for 1st IBM PC is major reason latest PCs still use 80x86 instruction set (Pentium 4); could still run program from 1981 PC today

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (22) Beamer, Summer 2007 © UCB Instructions as Numbers (1/2) Currently all data we work with is in words (32-bit blocks): Each register is a word. lw and sw both access memory one word at a time. So how do we represent instructions? Remember: Computer only understands 1s and 0s, so “ add $t0,$0,$0 ” is meaningless. MIPS wants simplicity: since data is in words, make instructions be words too

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (23) Beamer, Summer 2007 © UCB Instructions as Numbers (2/2) One word is 32 bits, so divide instruction word into “fields”. Each field tells computer something about instruction. We could define different fields for each instruction, but MIPS is based on simplicity, so define 3 basic types of instruction formats: R-format I-format J-format

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (24) Beamer, Summer 2007 © UCB Instruction Formats I-format: used for instructions with immediates, lw and sw (since the offset counts as an immediate), and the branches ( beq and bne ), (but not the shift instructions; later) J-format: used for j and jal R-format: used for all other instructions It will soon become clear why the instructions have been partitioned in this way.

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (25) Beamer, Summer 2007 © UCB R-Format Instructions (1/5) Define “fields” of the following number of bits each: = opcodersrtrdfunctshamt For simplicity, each field has a name: Important: On these slides and in book, each field is viewed as a 5- or 6- bit unsigned integer, not as part of a 32-bit integer. Consequence: 5-bit fields can represent any number 0-31, while 6-bit fields can represent any number 0-63.

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (26) Beamer, Summer 2007 © UCB R-Format Instructions (2/5) What do these field integer values tell us? opcode : partially specifies what instruction it is -Note: This number is equal to 0 for all R-Format instructions. funct : combined with opcode, this number exactly specifies the instruction Question: Why aren’t opcode and funct a single 12-bit field? -Answer: We’ll answer this later.

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (27) Beamer, Summer 2007 © UCB R-Format Instructions (3/5) More fields: rs (Source Register): generally used to specify register containing first operand rt (Target Register): generally used to specify register containing second operand (note that name is misleading) rd (Destination Register): generally used to specify register which will receive result of computation

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (28) Beamer, Summer 2007 © UCB R-Format Instructions (4/5) Notes about register fields: Each register field is exactly 5 bits, which means that it can specify any unsigned integer in the range Each of these fields specifies one of the 32 registers by number. The word “generally” was used because there are exceptions that we’ll see later. E.g., -mult and div have nothing important in the rd field since the dest registers are hi and lo -mfhi and mflo have nothing important in the rs and rt fields since the source is determined by the instruction (p. 264 P&H)

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (29) Beamer, Summer 2007 © UCB R-Format Instructions (5/5) Final field: shamt : This field contains the amount a shift instruction will shift by. Shifting a 32-bit word by more than 31 is useless, so this field is only 5 bits (so it can represent the numbers 0-31). This field is set to 0 in all but the shift instructions. For a detailed description of field usage for each instruction, see green insert in COD 3/e (You can bring with you to all exams)

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (30) Beamer, Summer 2007 © UCB R-Format Example (1/2) MIPS Instruction: add $8,$9,$10 opcode = 0 (look up in table in book) funct = 32 (look up in table in book) rd = 8 (destination) rs = 9 (first operand) rt = 10 (second operand) shamt = 0 (not a shift)

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (31) Beamer, Summer 2007 © UCB R-Format Example (2/2) MIPS Instruction: add $8,$9,$ Binary number per field representation: Called a Machine Language Instruction Decimal number per field representation: hex representation: 012A 4020 hex decimal representation: 19,546,144 ten hex

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (32) Beamer, Summer 2007 © UCB I-Format Instructions (1/4) What about instructions with immediates? 5-bit field only represents numbers up to the value 31: immediates may be much larger than this Ideally, MIPS would have only one instruction format (for simplicity): unfortunately, we need to compromise Define new instruction format that is partially consistent with R-format: First notice that, if instruction has immediate, then it uses at most 2 registers.

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (33) Beamer, Summer 2007 © UCB I-Format Instructions (2/4) Define “fields” of the following number of bits each: = 32 bits opcodersrtimmediate Again, each field has a name: Key Concept: Only one field is inconsistent with R-format. Most importantly, opcode is still in same location.

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (34) Beamer, Summer 2007 © UCB I-Format Instructions (3/4) What do these fields mean? opcode : same as before except that, since there’s no funct field, opcode uniquely specifies an instruction in I-format This also answers question of why R-format has two 6-bit fields to identify instruction instead of a single 12-bit field: in order to be consistent with other formats. rs : specifies the only register operand (if there is one) rt : specifies register which will receive result of computation (this is why it’s called the target register “rt”)

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (35) Beamer, Summer 2007 © UCB I-Format Instructions (4/4) The Immediate Field: addi, slti, sltiu, the immediate is sign-extended to 32 bits. Thus, it’s treated as a signed integer. 16 bits  can be used to represent immediate up to 2 16 different values This is large enough to handle the offset in a typical lw or sw, plus a vast majority of values that will be used in the slti instruction. We’ll see what to do when the number is too big in our next lecture…

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (36) Beamer, Summer 2007 © UCB I-Format Example (1/2) MIPS Instruction: addi $21,$22,-50 opcode = 8 (look up in table in book) rs = 22 (register containing operand) rt = 21 (target register) immediate = -50 (by default, this is decimal)

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (37) Beamer, Summer 2007 © UCB I-Format Example (2/2) MIPS Instruction: addi $21,$22, Decimal/field representation: Binary/field representation: hexadecimal representation: 22D5 FFCE hex decimal representation: 584,449,998 ten

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (38) Beamer, Summer 2007 © UCB Peer Instruction Which instruction has same representation as 35 ten ? 1. add $0, $0, $0 2. subu $s0,$s0,$s0 3. lw $0, 0($0) 4. addi $0, $0, subu $0, $0, $0 6. Trick question! Instructions are not numbers Registers numbers and names: 0: $0,.. 8: $t0, 9:$t1,..15: $t7, 16: $s0, 17: $s1,.. 23: $s7 Opcodes and function fields (if necessary) add : opcode = 0, funct = 32 subu : opcode = 0, funct = 35 addi : opcode = 8 lw : opcode = 35 opcodersrtoffset rdfunctshamt opcodersrt opcodersrtimmediate rdfunctshamt opcodersrt rdfunctshamt opcodersrt

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (39) Beamer, Summer 2007 © UCB Peer Instruction Answer Which instruction bit pattern = number 35? 1. add $0, $0, $0 2. subu $s0,$s0,$s0 3. lw $0, 0($0) 4. addi $0, $0, subu $0, $0, $0 6. Trick question! Instructions != numbers Registers numbers and names: 0: $0, …, 8: $t0, 9:$t1, …,16: $s0, 17: $s1, …, Opcodes and function fields add : opcode = 0, function field = 32 subu : opcode = 0, function field = 35 addi : opcode = 8 lw : opcode =

CS61C L9 MIPS Logical & Shift Ops, and Instruction Representation I (40) Beamer, Summer 2007 © UCB In conclusion… Logical and Shift Instructions Operate on individual bits (arithmetic operate on entire word) Use to isolate fields, either by masking or by shifting back & forth Use shift left logical, sll, for multiplication by powers of 2 Use shift right arithmetic, sra, for division by powers of 2 Simplifying MIPS: Define instructions to be same size as data word (one word) so that they can use the same memory (compiler can use lw and sw ). Computer actually stores programs as a series of these 32-bit numbers. MIPS Machine Language Instruction: 32 bits representing a single instruction opcodersrtimmediate opcodersrtrdfunctshamt R I J target address opcode